Long black tie wraps.
One on each side.
Attach to spring clip and around black rubber mount.
Cut extra off tie wrap off.
It is well hidden and difficult to notice because it blends into the rubber mount, requires both tie wraps/sides to be cut for removal. It does take an extra few minutes to remove and you need either a very skinny wire cutter or if placed extra tight requires a heated (red hot) small flat head screw driver to melt zip tie.
Similar can be done with the harley detachable backrest with placement of tie wrap/zip tie under the button you press on latch.
Cheap.

FYI - Our ShieldShackle is currently out of stock for the Road Kings, but available for the Softails. It should be back in stock by the 2nd week of February, and we are currently offering free shipping through the end of the month. So you might want to take advantage of the free shipping AND the HDForums discount, even though your product won't ship for a couple of weeks.